1. Field of the Invention
The invention relates to a speed control circuit arrangement for an AC commutator series motor and, more particularly, to an improvement over the speed control circuit arrangement in which an inductive voltage (hereinafter referred to as a flash voltage) regenerated in the armature of the motor is positively fed back to an electric charge storing capacitor through a resistor, that is, with such a polarity that it may increase a control voltage and the speed control characteristic at a low speed running of the motor may be improved.
2. Description of Prior Art
FIG. 1 shows a circuit diagram of a conventional speed control circuit arrangement. In the FIGURE, reference numeral 1 designates a power source switch; 2 a Triac; 3 a Diac for supplying a trigger signal to the gate etectrode of the Triac; 4 an electric charge storing capacitor for causing the Diac 3 to produce the trigger pulse; 5 a protective resistor for preventing an overcurrent from flowing into the Diac 3; 6 a variable resistor for adjusting the rotational speed of the motor from the minimum to the maximum speed; 7 an AC commutator motor. In operation, the power switch 1 is first turned on. Upon the turning on, a positive half cycle of voltage, for example, is supplied from the AC power source AC to the power source switch side. At this time, voltage is applied across the charge storing capacitor 4 through the protective resistor 5 and the variable resistor 6. As a result, the capacitor 4 is charged at a time constant determined by the resistances of the resistors 5 and 6 and the capacitance of the capacitor 4. When the variable resistor 6 is set at the maximum resistance value, the voltage at the terminal of the capacitor 4 connecting to the Diac 3 does not reach the breakover voltage of the Diac 3, so that the Diac does not conduct and no trigger signal is applied to the gate electrode of the Triac 2. Therefore, the Triac is not turned on and thus the motor does not operate. Then, the resistance of the variable resistor 6 is gradually decreased by adjusting the variable resistor 6. With decreasing of the resistance, the charging voltage increases to reach the breakover voltage of the Diac 3. At this time, the AC power source A.C. applies a positive half cycle of voltage across the motor 7, with the result that an exciting current flows into a field coil and an armature A starts rotating. At that time, however, the time constant is large so that the conducting phase angle of the Triac is retarded by relatively large amount with respect to the phase of the power source voltage. Accordingly, the motor initiates to rotate from low speed. As the resistance of the variable resistor 6 is decreased, the rotational speed of the motor increases and when the resistance is adjusted for the minimum value, the rotational speed reaches the maximum.
However, since the speed control circuit arrangement has the following defect, practically it has not been used but the DC speed control circuit arrangement has been used instead. The waveforms shown in FIGS. 4A and 4B are those of voltages appearing between terminals A and B of the motor shown in FIG. 1. FIG. 4A is the voltage waveform when the motor is connected to a light load and FIG. 4B a voltage waveform when the motor is connected to a heavy load. As seen from those waveforms, the conducting phase angle changes depending on the load. More specifically, the conducting phase angle with respect to the heavy load is more retarded than that with respect to the light load. Accordingly, with increase of load, the rotational speed of the motor decreases and electric power injected into the motor also decreases. This further decreases the rotational speed. Thus, the operation of the motor at low speed becomes very unstable. The reason to cause such a phenomena will be described in brief below. The shaded portions in the waveforms of FIGS. 4A and 4B indicate the flash voltages induced in the armature of the motor when the drive current to the motor is shut off by the Triac 3. The flash voltage depends largely on the rotational speed of the motor. As the rotational speed increases, the flash voltage decreases. On the other hand, as the rotational speed decreases, the flash voltage increases. In this way, the flash voltage changes in accordance with the rotational speed. This is seen from comparison of the respective waveforms between the respective light and heavy loads, as illustrated in FIGS. 4A and 4B. As shown, the flash voltage is small at the light load (the motor speed is high) while it is large at the heavy load (the motor speed is low). It is considered that the flash voltage causes the above-mentioned problem. Namely, the flash voltage is opposite in polarity to the voltage of the preceding half cycle and is the same in polarity as the voltage of the succeeding half cycle. However, the inductive current flowing through the capacitor 4 due to the flash voltage is opposite in polarity to the current caused by the succeeding half cycle voltage. This will be further eleborated referring again to the circuit diagram shown in FIG. 1. The flash voltage appears between the terminals A and B of the armature A of the motor 7. Assume now that the preceding half cycle voltage is positive and causes a current to flow from the terminal A to B. In this case, the terminal A is positive and the B is negative. When the positive half cycle voltage becomes zero and the drive current is shut off, a flash voltage appears with a polarity that A is negative and B is positive. The current caused by the flash voltage will flow from A to B. In the succeeding negative half cycle, the power source boltage starts increasing from zero with a polarity that B is positive and A is negative. The power source voltage causes a current to flow through a path of the terminals B, A, the capacitor 4, the variable resistor 6, the resistor 5 to power switch 1. Accordingly, the capacitor is charged by a potential difference beween the power source voltage and the flash voltage. For example, when the load of the motor increases and the rotational speed decreases, the flash voltage increases. Accordingly, the arrival of the voltage across the capacitor at the breakover voltage will be delayed so that the conducting phase angle will decrease and therefore the drive current flowing into the motor 7 will decrease. As a result, the motor speed will further decrease. On the other hand, when the load decreases, the motor speed increases and therefore the flash voltage also decreases. Hence, the conducting phase angle advances to increase the current flowing through the motor 7 and the motor speed. Through this vicious cycle, the motor speed rapidly increases. As described above, when the conventional motor speed control circuit arrangement is used, it is impossible to secure a stable operation of motor at low speed.